This invention relates to a square-wave current generator for supplying an excitation coil for producing a magnetic field with a fixed exciting current that periodically reverses its direction, and more specifically to a square-wave current generator adapted for use with an electromagnetic flow meter employing a square-wave excitation system.
Conventionally known are electromagnetic flow meters which use a square-wave excitation system. In one such prior art electromagnetic flow meter, a magnetic field of a fixed magnitude, changing its polarity with a given period, is applied substantially at right angles to the flow direction of a conductive fluid, a voltage produced in the fluid is detected by means of a pair of electrodes, and the flow rate of the fluid is measured on the basis of the detection value thus obtained. In the electromagnetic flow meter of this type, the direction of the magnetic field and hence the detected voltage is periodically reversed, so that the detected voltage value is less liable to electrochemical noise attributed to unidirectional interaction between the electrodes and the fluid. Moreover, noise produced by the change of interlinkage between an electric circuit formed of the electrodes and the fluid and magnetic flux forming the magnetic field can be reduced by measuring the exciting current while its amplitude is substantially fixed because the exciting current is a square-wave current. Furthermore, noise attributable to stray capacitance in the wires and the excitation coil interlinked to the electrodes can be lowered. Owing to these advantages, the electromagnetic flow meter is capable of stable flow rate measurement.
The prior art square-wave current generator with these advantages is generally constructed as follows. A DC power supply is connected through a constant-current circuit to the input terminals of four switching transistors which constitute a bridge circuit. Connected to the output terminals of the transistors is an excitation coil through which an exciting current in the form of a symmetrical square wave flows. Positive and negative voltage outputs from the symmetrical square-wave current generator are applied alternately to the two opposite pairs of transistors in the bridge circuit, and an exciting current reversing with a given period and having equal positive and negative amplitudes flows through the excitation coil. Connected between the DC power supply and the bridge circuit are a resistor for exciting current detection and a transistor for constant-current control to keep the exciting current at a predetermined value. Current flowing through the constant-current control transistor is controlled by the output of an operational amplifier. The input of the operational amplifier is applied to a DC reference voltage signal delivered from a reference signal generator and a voltage drop caused by the current flowing through the detecting resistor, and the amplifier supplies the constant-current control transistor with a base current equivalent to the difference between the two inputs. Actuated by the base current, the constant-current control transistor adjusts the exciting current so that the two inputs applied to the operational amplifier are equalized. Thus, the direction and amplitude of the exciting current are made to correspond to the reference voltage signal and the voltage signal from the symmetrical square-wave current generator.
In the square-wave current generator of the aforementioned construction, a constant current can be made to flow in two directions by the use of a single power supply. However, this generator still is subject to the following drawbacks. The base currents of those two transistors among the four transistors of the bridge circuit which are on the side of the detecting resistor and the constant-current control transistor flow not through the excitation coil but through the detecting resistor. Accordingly, there will be an error or a discrepancy between the amplitudes of the exciting current and the reference voltage signal. Also, leakage currents at the four transistors forming the bridge circuit will cause errors. Among these errors, the error attributable to the base currents can be removed by the use of MOSFETs. The presently available FETs, however, require an additional power supply, as well as the DC power supply, to obtain positive and negative exciting currents. Moreover, the prior art generator requires a substantial number of transistors including the four transistors used in the bridge circuit and the constant-current control transistor. The four transistors of the bridge circuit must be of a high-power type for the switching of the exciting current. Since the exciting current in operation continually flows through the two of the four transistors of the bridge circuit and the constant-current control transistor, a lot of electric power is consumed by these three transistors. This makes the transistors, and therefore the whole circuit, less reliable.